Smoke detectors of the photo-electric type utilize a focused light beam passing through a volume which receives smoke from the ambient atmosphere, with a photo-responsive device viewing the light beam at an angle to the axis thereof, so that smoke particles illuminated by the beam are seen by the photo-responsive device.
Such detectors are calibrated to provide an alarm when the concentration of smoke in the light beam reaches a predetermined level. Certain organizations that test and approve smoke detectors require that means be provided whereby the user of the detector can test the operability thereof, by simulating the amount of smoke to which the detector is required to respond.
Various methods have been proposed to provide such test means. However, it has been found difficult to provide a method which will give consistent results in a smoke detector manufactured in great quantity because of the difficulty of holding accurate tolerances in the mechanical components. Methods used heretofore depend on light reflected or scattered from an object, such as a wire, inserted into the light beam. However, the amount of light scattered onto the cell depends on the size of the wire, the surface finish thereof, and its position in the light beam. The diameter of the wire required to provide the small amount of light required is very small, and is therefore susceptible to damage in handling during manufacture or damage in use by the user.